For centuries various types of wind power generators have been built to convert the kinetic energy of the wind into useful energy or power. In recent years many worthwhile wind generators have been built utilizing the latest developments in aerodynamics, products and manufacturing methods. These wind generators are generally classified as either horizontal or vertical axis machines.
Horizontal axis machines have been built from small sizes, such as farm windmills, to much larger sizes such as the 200 kilowatt machine at Clayton, N. Mex. which has a rotor 125 feet in diameter. Such horizontal axis machines utilize aerodynamic lift forces to cause rotation of their vanes or airfoils and generate their maximum power when the vanes are traveling at 2 to 16 times the speed of the wind. Even with this high vane speed, a large horizontal axis machine usually utilizes a gear box in order to drive an electrical generator at a higher and synchronous speed, such that output frequency of the generated power may be synchronized with, and electrically connected to, a 60 cycles per second power system. Because of the necessity to drive the electrical generator at its synchronous speed, and simultaneously control the amount of power developed by the rotor of the wind generator, such a large wind generator is usually equipped with a vane pitch control mechanism to control the angle of attack of the vanes with respect to the wind. Also, such a large wind generator is usually equipped with a yaw control mechanism to keep the entire rotor facing into the wind. An important additional purpose of these two control mechanisms is to slow down or stop the rotor, and thus prevent damage attributable to over-speed, if excessively high winds should occur. Since the rotor mechanism, gear box, pitch and yaw control mechanisms must be mounted upon a structure sufficiently high that the rotor vane tips will clear the ground, it is quite evident that such a large horizontal axis wind generator can be quite complicated and expensive to build.
Vertical axis wind generators are generally sub-classified as either drag or lift type machines depending upon whether their rotor vanes utilize drag forces or aerodynamic lift forces to cause rotor rotation. A drag type machine harnesses the component of the wind force perpendicular to the surfaces of its vanes, similarly to the manner in which a flat object held perpendicular to the wind is pushed along by the wind. The rotor vanes of drag type machines are usually oriented relatively flat to the wind while they are traveling in the direction of the wind and producing power and are oriented parallel to the wind while they are traveling against the direction of the wind so as not to seriously impede power generation. The rotors of such machines generally have low energy conversion efficiencies since only the vanes which are traveling with the wind are contributing rotary force or power while those vanes which are traveling against the wind are hindering rotor rotation and power generation. An example of a drag type vertical axis wind generator is shown in Crehore U.S. Pat. No. 4,184,084. A particular disadvantage of a typical drag type wind generator is that it develops its maximum useable power at a low rotary speed of its rotor, typically while the rotor vanes are traveling at one-third to one-half the speed of the wind, thus it is usually necessary to use a gear box with a greater speed ratio in order to obtain higher output speeds. Although drag type wind generators have the advantage of being self-starting, control of their speed while they are operating under varying wind speed conditions, or to prevent over-speeding when high winds occur, can be quite difficult.
Lift type vertical axis wind generators have rotors with airfoil shaped vanes which utilize aerodynamic lift forces to cause rotor rotation, and thus generate their maximum power while the vanes are traveling at 2 to 16 times the speed of the wind. The aerodynamic lift forces and vane speed range for maximum power generation are similar to those previously described for horizontal axis wind generators. However a vertical axis wind generator has a rotor which revolves about a vertical axis, thus its vanes travel in an orbital manner about the vertical axis. Consequently the vanes are effectively traveling with the wind during one-half of each revolution of the rotor and are effectively traveling against the wind during the other one-half of each revolution of the rotor. Many problems have been encountered with vertical axis lift type machines which have limited their widespread use. For example, since their airfoil shaped vanes travel at a relatively high rate of speed and their vanes must continuously be oriented at an effective angle of attack with respect to the wind, they are difficult to control under varying or gusty wind conditions. Fixed-vane machines such as the "eggbeater style" Darrieus wind power generator, as described in The Wind Power Book by Jack Park, are easily stalled when overloaded or in the event of a sudden increase in wind speed while a load is applied to the rotor. Also, this type of rotor normally is not self-starting and must be accelerated by another power source up to a speed at which the vanes become sufficiently effective to cause self-acceleration of the rotor. Variable-pitch vane machines have been developed to overcome these problems, such as the Giromill which was undergoing development tests in 1982 at the Rocky Flats Wind Energy Research Center near Golden, Colo. There are many patented devices for the purpose of vane pitch control involving cams, linkages and servo-mechanisms. The two principal purposes of these devices are, firstly, to make the rotor self-starting and, secondly, once the rotor is up to its normal operating speed, to control the angle of attack of each vane in a cyclical manner such as by rocking each vane back and forth during each revolution of the rotor, so as to optimize the aerodynamic lift force developed by each vane during each complete revolution of the rotor. Since it is necessary to control all of the vanes simultaneously, such multi-vane cyclical devices can be complicated, expensive and subject to breakdown. Thus there remains a need for an inexpensive, reliable and effective means for vane pitch control on lift type vertical axis wind generators.
Although each of the above described machines, and patents referred to herein, describe machines for producing useable power from the wind, none of them have been able to produce power on an economically feasible basis. Each of them has one or more deficiencies which are partly attributable to the unpredictability of the wind, and partly due to the construction and consequent wind energy conversion characteristics of the wind generator itself.
One apparent advantage a vertical axis wind generator has, as compared to a horizontal axis wind generator, is that its vertical power shaft may be more conveniently coupled to a load device, such as an electrical generator, which is located near the ground rather than upon a high structure. Another apparent advantage is that its own shaft also acts as its principal support structure, thus eliminating the need for a separate support structure at many wind generator installation sites.